The present invention relates to a jet-propulsion watercraft which rearwardly ejects water pressurized and accelerated by a water jet pump and planes on a water surface as the resulting reaction. More particularly, the present invention relates to an improved structure of the water jet pump.
In recent years, so-called jet-propulsion personal watercraft (PWC) have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is configured to have a water jet pump driven by an engine, for pressurizing and accelerating water sucked from a water intake generally provided on a hull bottom surface and ejecting it rearward from an outlet port. Thereby, the personal watercraft is propelled.
The water jet pump is mainly comprised of a cylindrical pump casing and an impeller supported coaxially in the pump casing and rotatably driven by the engine. In general, the pump casing is made of aluminum alloy for weight saving and the impeller is made of stainless steel highly resistant to cavitations. For this reason, a cylindrical sleeve made of stainless steel resistant to wear is fitted tightly into an inner peripheral face of the pump casing along which the hard impeller slides, thereby reducing wear which is caused by the impeller.
However, since the sleeve is made of stainless steel equal in hardness to the stainless steel of the impeller, the pump casing made of aluminum alloy having an ionization tendency greater than that of the stainless steel tends to electrolytically corrode, i.e., oxidize, especially when the personal watercraft is in contact with water. Further, with use, the oxidation of the pump casing is progressed and thereby the pump casing has an increased volume or is expanded. As a result, the pump casing inwardly pushes the sleeve, thereby causing an increase in frictional resistance between the sleeve and the impeller, which leads to their wear. This wear may reduce the lives of parts of the watercraft as well as generates sliding noises.
The present invention addresses the above-described condition, and an object of the preset invention is to provide a pump structure of a jet-propulsion watercraft capable of preventing an increase in frictional resistance between a sleeve and an impeller by forming an additional layer made of a material that prevents electrolytic corrosion or oxidation between a pump casing and the sleeve, which are made of different metals.
In accordance with the present invention, there is provided a pump structure of a jet-propulsion watercraft adapted to eject water pressurized and accelerated by a water jet pump driven by an engine from an outlet port so as to be propelled as the resulting reaction, the water jet pump comprising a pump casing having a cylindrical inner peripheral face, and an impeller supported in the pump casing coaxially with the inner peripheral face of the pump casing and rotatably driven by the engine, and a cylindrical sleeve provided between the inner peripheral face of the pump casing and an outer periphery of the impeller and made of a metal different from a metal of the pump casing, wherein an annular layer made of a material that subdues electrolytic corrosion or oxidation of the pump casing or the sleeve is formed between the pump casing and the sleeve.
In accordance with the above invention, since the layer made of the material that subdues electrolytic corrosion or oxidation is formed between the pump casing and the sleeve, which are made of different metals, the increase in the frictional resistance between the sleeve and the impeller can be prevented, and consequently, their wear, and sliding noises and reduction of lives of parts caused by the wear can bedecreased.
With the above configuration, for example, when a setting is made such that the impeller is changed into another impeller with a different outer diameter, only changing the associated sleeve into another sleeve according to the outer diameter of the another impeller, is needed, but changing or processing the pump casing is not necessary. In this case, the thickness of the layer may be changed in accordance with the change of the outer diameter of the sleeve.
The annular layer may be made of a material having an ionization tendency more than that of the pump casing or the sleeve, or otherwise a non-conductive material.
When aluminum alloy is used for the pump casing and stainless steel is used for the sleeve, the annular layer may be made of a metal having an ionization tendency more than that of an aluminum alloy having an ionization tendency more than that of stainless steel, for example, magnesium alloy. Alternatively, the annular layer may be made of a metal having an ionization tendency more than that of the pump casing made of at least an aluminum alloy, for example, zinc alloy. As the aluminum alloy, an aluminum alloy having an ionization tendency less than that of zinc is preferably used. Thereby, the above-mentioned inward expansion of the pump casing can be subdued. Therefore, the inward push against the sleeve and the resulting increase in the frictional resistance between the sleeve and the impeller can be subdued.
As a matter of course, other than metal such as magnesium alloy and zinc alloy, electrically insulating materials, for example, synthetic resin, which substantially impedes ion exchange between the pump casing and the sleeve, may be used as the annular layer. Thus, the use of a plastic material such as resin for the annular layer allows interference between the sleeve and the impeller to be absorbed by elastic deformation of the annular layer. This makes it easy to keep a clearance between the sleeve and the impeller.
When resin is used for the annular layer, powdered metal having an ionization tendency which is, for example, at least more than that of the material of the pump casing, may be mixed with the resin.
The annular layer may be coated over the surface (e.g., outer peripheral face) of the sleeve, which is smaller in size and easier to process than the pump casing, by a process method such as coating or plating. However, if the pump casing is coated, the pump casing can be coated over the inner peripheral face thereof by a similar process method.
The annular layer may coated over an end face of the sleeve, in particular, on the side of the sleeve fitted into the pump casing, as well as the outer peripheral face of the sleeve, for isolating the pump casing from the sleeve. Thereby, the pump casing and the sleeve can be reliably insulated.
As a matter of course, the engine may be a two-cycle engine or a four-cycle engine.